What stops underleveled players from 'leeching' in high torment levels?

Solution 1:

Nothing at all. What metric can be used to determine if someone is leeching? There's at least one build I know of that can't kill anything at all by itself, but is invaluable in rift efficiency. If you used kills as a metric, that build would be completely useless, as its pure support. No one would run with it if they didn't get the same drop rates as those doing the killing.

And what about partying with friends? If you can handle T6 on your own, maybe you want to invite a friend along, to help him get better gear faster.

Essentially, Blizzard wants you to play with other people. Doing anything at all to penalize what, in some cases, can be construed as leeching, would run counter to that ethos.

You can run what you want, with whom you want, at any difficulty you want, and everyone in the game will have the same drop rates (so long as you're in the kill radius, anyways), regardless of whether or not they're contributing or not.